Lithium ion battery heater systems and methods

ABSTRACT

A battery heater system for a battery used in cold weather operations and methods for using the battery heater system are described. Embodiments of the battery heater system may incorporate a heater switch with an indicator, a timer circuit, a controller, a voltage meter, a temperature transducer, and a heating element. In some methods of using the device, the battery powers the heating element for a fixed cycle time based on the time to discharge the battery at a cold-soaked temperature. In other methods of using the device, the battery powers the heating element for a varying cycle time as necessary to discharge the battery to a discharge cut-off voltage value. In other methods of using the device, the heating element is operated using a duty cycle that is varied based on the battery temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Patent Application Ser. No.61/892,801 filed Oct. 18, 2013, the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The battery heater systems and methods described herein are in the fieldof battery devices. More specifically, the systems and methods disclosedherein are in the field of heaters for batteries for use in anyaircraft, vehicle, mobile, or stationary devices exposed to extendedcold temperatures. The systems and methods related also to aircraftbatteries and internal heaters that are integrated into aircraftbatteries. The systems and methods also relate to a self-powered batteryheater system where the battery heater system is powered by the batteryitself.

Batteries used to start vehicles such as aircraft and other heavyequipment may be required to produce significant current to support thestarting and operation of the vehicle in which they are installed.Similarly, batteries used to power stationary or mobile devices, such ascomputers, laptops, mobile phones, and other electronic equipment, mayneed to operate in cold environments. When the vehicles or other devicesare located or operated in cold environments, the batteries may becomecold-soaked to the ambient temperature of the environment during periodswhen the vehicle is not in use. In some cases aircraft may be exposed totemperatures down to −45 degrees C. for extended periods. Batteries atthese cold temperatures may be incapable of performing properly if theyare used to attempt to start an engine, for example, because of frozenelectrolyte or other effects of the low temperature.

Typically, when an aircraft will be exposed to such low temperatures foran extended time, the batteries may need to be removed from the aircraftand placed in a temperature-controlled environment to preventelectrolyte freezing. Aircraft batteries may be difficult to remove andreinstall in an aircraft, especially by personnel wearing cold weatherclothing and gloves as would typically be required.

If it is impractical to remove the battery, the battery will get so coldthat its electrolyte will freeze. When frozen, the electrolyte'sviscosity will increase and will create a high internal resistance inthe battery which resists the flow of Lithium ions and electronsnecessary to sustain a high voltage and continuous current needed tostart an aircraft engine or power a heater. In such situations, anexternal power source, such as a ground power cart, is necessary toprovide electrical power to operate heaters provided for the battery,which may be internal heaters. Such external power sources may not beavailable at small airports or remote cold weather locations. If anexternal power source with a higher sustained nominal voltage (e.g. 28V)is available, the battery heater system will not need to employ anytime, voltage, or duty cycle-based algorithm and can operate normally atfull power up to the target temperature.

The dependence on either preventing electrolyte freezing or an externalpower sources is due to the assumption that the heater will operate at aconsistent power level at its nominal voltage and current requirements.Furthermore the heater is operated using control logic focused onreaching and maintaining a target temperature. The inventive heatersystem and methods described herein are designed to use the limitedpower available in the cold batteries to self-heat.

BRIEF SUMMARY OF THE INVENTION

The new devices and methods described herein provide an improved batterycapable of using its own electrochemical cells with frozen or coldelectrolyte to discharge electrical energy in a manner that enables itsheating elements to warm up the cold soaked battery. A battery with thisbattery heater system will not be required to be removed from anaircraft in a cold temperature environment for room temperature storage,nor will it be dependent on external power to preheat the battery.

In some embodiments of the invention, the method for operating anelectrical heater to heat one or more electrochemical cells disposed inthe battery case comprises the steps of determining a power-on timeperiod and a power-off time period, discharging the electrochemicalcells through the electrical heater for the power-on time period, anddisconnecting the electrochemical cells from the electrical heater forthe power-off time period. The battery is heated by sequentiallydischarging the electrochemical cells through the heater followed bydisconnecting the heater from the electrochemical cells to allow thebattery voltage to recover, and those processes are repeated until thetemperature of the electrochemical cells are at or above a targettemperature.

In some embodiments, determining a power-on time period is done bydischarging the electrochemical cells through the electrical heateruntil the voltage of the electrochemical cells drops to a dischargecut-off voltage and measuring the time period elapsed until the voltageof the electrochemical cells dropped to the discharge cut-off voltage.

In some embodiments, determining a power-off time period is performed bydisconnecting the electrochemical cells from the electrical heater whenthe voltage of the electrochemical cells drops to the discharge cut-offvoltage and then measuring the time period elapsed until the voltage ofthe electrochemical cells recovers to its open circuit voltage.

In some embodiments, the power-on time period and the power-off timeperiod are determined when the temperature of the electrochemical cellsare a selected temperature, such as a temperature of approximately −40degrees C. In some embodiments the time periods are determined using atest article of the battery and those time periods are then pre-definedfor use in the production batteries used in actual aircraft. In otherembodiments, the determination of the power-on time period and thepower-off time period are executed on the battery in situ.

The discharge cut-off voltage use to find the power-on and power-offtime periods is selected to prevent damage to the electrochemical cellscaused by overdischarging the battery at cold temperatures. The targettemperature for the device to stop using the heater to heat theelectrochemical cells may be greater than the melting point of theelectrolyte utilized in the electrochemical cells.

In other embodiments of the method, the electrical heater is used toheat one or more electrochemical cells disposed in the battery case byselecting an initial duty cycle, and repeatedly discharging theelectrochemical cells through the electrical heater for the duty cycle,and modifying the percentage of the duty cycle as the temperature of theelectrochemical cells increase from the applied heat. In someembodiments of this method, the duty cycle is modified based on thetemperature of the electrochemical cells by monitoring the temperatureof the electrochemical cells, and increasing the duty cycle to a highervalue when the temperature of the electrochemical cells exceeds athreshold temperature. Then, after the electrochemical cells reaches thefinal threshold temperature, the duty cycle is decreased or terminated.In some embodiments of the method, the duty cycle may be increased to asecond higher value when the temperature of the electrochemical cellsexceeds a second higher threshold temperature. The second thresholdtemperature may be equal to or greater than the melting temperature ofthe electrolyte in the electrochemical cells.

After the electrochemical cells reach the final threshold temperature,the duty cycle may be reduced by an incremental value each time thetemperature of the electrochemical cells increases by an incrementalvalue. The incremental value by which the duty cycle is reduced may be10% in some embodiments. The second incremental value by which thetemperature increase is measured may be 1 degree C. in certainembodiments.

In some other embodiments, the duty cycle is selected by determining apower-on time period and a power-off time period, and then calculatingthe duty cycle as the ratio of the power-on time period to the sum ofthe power-on time period and the power-off time period. As describedbefore, the power-on time period may be determined by discharging theelectrochemical cells through the heater and measuring the time requiredfor the battery voltage to fall to a discharge cut-off voltage value.Similarly, the power-off time period may be determined by disconnectingthe electrochemical cells from the electrical heater and measuring thetime period required for the voltage of the electrochemical cells torecover to its open circuit voltage. The determination of the power-ontime period and the power-off time period may be performed at a selectedtemperature or using a test article of the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a lithium ion battery with an embodimentof the heater installed in the battery.

FIG. 2 is an electrical schematic of an embodiment of the lithium ionbattery heater device.

FIG. 3A is a graph of the voltage and temperature of an exemplarylithium ion battery during operation of an embodiment of the lithium ionbattery heater system.

FIG. 3B is a diagram of an embodiment of a method of operating thelithium ion battery heater system.

FIG. 4A is a graph of the voltage and temperature of an exemplarylithium ion battery during operation of a second embodiment of thelithium ion battery heater system.

FIG. 4B is a diagram of an embodiment of a method of operating thelithium ion battery heater system.

FIG. 5A is a graph of the duty cycle of an exemplary lithium ion batteryduring operation of a third embodiment of the lithium ion battery heatersystem.

FIG. 5B is a diagram of an embodiment of a method of operating thelithium ion battery heater system.

FIG. 5C is a diagram of an embodiment of a method of operating thelithium ion battery heater system.

DETAILED DESCRIPTION OF THE INVENTION

The battery heater systems and methods described herein include abattery with an internal heating element, a temperature transducer, avoltage meter, and an electronic controller. The temperature transducerprovides a signal, whether analog or digital, representative of thetemperature of the battery. The voltage meter provides a measurement ofthe voltage between the terminals of the battery or any combination ofelectrochemical cells within the battery. The internal heating elementis an electrical-powered heating element, such as a resistive heatingelement, that is disposed within the battery case. The electroniccontroller is designed or programmed to execute one or more of theembodiments of the method of using the battery heater system.

Since, as described in the background section, the cold-soaked batteryis not capable of providing sufficient continuous current at anappropriate voltage to power the heating element, the controlleroperates the heating elements using the battery's power in anintermittent or pulsed manner. The pulses may be controlled in a varietyof ways and algorithms, such as a duty cycle, a fixed time durationdischarge, or a fixed voltage drop discharge, among others. At thebeginning of the battery heating operation, each pulse provides verylittle heating effect because of the condition of the battery. However,as the battery slowly warms it becomes more quickly able to providepower to the heating element in each pulse, and the heating effectproceeds at a faster rate.

The repetitive pulse discharge of the battery through the heatingelement eventually results in some warming and thawing of the batteryelectrolyte which then decreases the electrolyte's internal resistance.The lower internal resistance enables the electrochemical cell tomaintain higher voltages for a given current. This leads to a decreasein the voltage drop in each pulse. As the battery become capable ofdischarging at a higher voltage during each pulse at the same amplitudeof current, more power (current×voltage=power) is delivered to theheaters. As more power is discharged from the electrochemical cells, theelectron and lithium ion flows cause more internal heating of theelectrolyte, and there is more external heat delivered to warm theelectrochemical cells from the heating element on the cell cases. Therate of heat transfer into the electrolyte increases as the electrolytegets warmer. When the battery is warmed to certain temperatures, thebattery becomes capable of delivering more current while maintaining itsvoltage above the recommended minimum end point voltage/cut-off voltage.At different temperatures, the battery then becomes capable of providingelectrical energy to other loads such as aircraft avionics, starters formain engines or auxiliary power units, cabin amenities, or lights.

The device and methods allow a user to actuate the battery heater systemusing a simple switch or button. There is no need to remove or reinstallthe battery from the aircraft, or to provide an external power source topower a heating element in the battery. The inventive methods ofoperating the heater system, including the timer component, preventdamage to the cold battery as a result of discharging it for too longbefore it has warmed up sufficiently. The inventive methods also allowthe battery heater system to self-heat at temperatures where normalmethods would not function due to the drop in voltage caused by a fullpower discharge.

Referring now to FIG. 1, a perspective view of a lithium ion battery 100is depicted. The lithium ion battery 100 is typically provided with acase 102 for containing the electrochemical cells and other componentsof the battery 100. The case 102 is provided with an output receptacle104 or other means of electrically connecting the battery to theaircraft electrical systems. In some embodiments of the battery heatersystem, the case 102 is provided with a button 106 for manual activationof the heater, indicator light 108 as an indicator of the status of theheater operation, or both. In some embodiments, the button 106 and light108 may comprise a single component serving both functions. In otherembodiments no button 106 or light 108 may be provided, and other meansof activating the heater may be provided such as an electronicallyactivated heater.

Referring now to FIG. 2, an electrical schematic of an embodiment of thebattery heater system 201 inside a lithium ion battery 203 is depicted.The battery is provided with temperature electronic controller 200,embodiments of which may comprise an integrated circuit running softwaredeveloped to embody the methods described herein. In other embodiments,the control electronics may comprise analog or digital hardwarecomponents combined to embody the methods described herein.

A switch 202 is provided for actuating the battery heater system. Asdescribed in relation to FIG. 1 in various embodiments of the heater,switch 202 may comprise a button 106 allowing a user to manually engagethe heater by depressing the button 106. In other embodiments, switch202 may comprise an electronic switch capable of being actuatedelectronically. The method of actuating switch 202 is not limiting ofthe invention as switch capable of actuating the circuit may be used invarying embodiments of the heater.

A timer 204 is provided for limiting the length of time that the batteryheater system remains in operation. This is intended to prevent theaccidental draining of the battery by lengthy heating of the battery. Atthe end of the prescribed time period, the timer 204 shuts off thebattery heater system to prevent unnecessary continued heating of thebattery when it is not required by the user. The length of the time mayvary depending on the specific characteristics of the battery and theconditions under which the heater will be used. In some embodiments thetimer 204 may limit the battery heater system to 2 hours of continuousoperation. The operation of timer 204 is not depicted in FIGS. 3B, 4B,5B, and 5C but the timer does limit the length of operation of themethods depicted in those figures.

A status indicator 206 may be provided for indicating that the batteryheater system is in operation. In some embodiments status indicator 206may be a light 108. In other embodiments, no status indicator may beprovided, or the indicator may comprise an indicator on a remote controlpanel outside the battery. Similarly, in other embodiments electroniccontroller 200 may provide a status indicator 206 via a data connectionto the avionics of the aircraft.

Once the battery heater switch is actuated, electronic controller 200initiates the operation of the battery heater system. The electroniccontroller 200 utilizes a temperature transducer 208 to monitor thetemperature of the electrochemical cells 210 in the battery. A voltagemeter 212 is also provided to measure the voltage between the terminalsof the battery or any combination of electrochemical cells. In variousembodiments, electronic controller 200 may utilize the voltage measuredby meter 212 in controlling the operation of the heater.

One or more heating elements 214 are provided to heat theelectrochemical cells 210 using electrical power from theelectrochemical cells 210. In the embodiment depicted if FIG. 2, only asingle heating element 214 is depicted, though depending on theconfiguration of a particular embodiment, multiple heating elements 214may be provided. The heating element 214 is activated and poweredthrough electronic controller 200. The heating elements 214 generateheat from electrical energy stored in the electrochemical cells 210.

Electronic controller 200 may access measurements of the batterytemperature from transducer 208, and the voltage of the batteryterminals from voltage meter 212. The control electronics also turns theheating element 214 on and off by applying voltage to it or removing thevoltage from it. The voltage and resulting current is provided by theelectrochemical cells 210. As described above, if the electrochemicalcells 210 are cold, improper heating of the electrochemical cells 210using heating element 214 may occur since the battery voltage willquickly fall below its discharge cut-off voltage without having producedany internal or external heat in the electrochemical cells 210. However,in various embodiments of the battery heater system, the electroniccontroller 200 is programmed or designed to heat the electrochemicalcells 210 in a manner that will not cause the battery voltage to fallbelow its discharge cut-off voltage, but will enable it to maintain thebattery voltage above its discharge cut-off voltage sufficiently to heatthe electrochemical cells 210 so that the battery is still operationalto provide sufficient current to start the airplane engines.

Referring now to FIGS. 3A and 4A, the operation of various embodimentsof the battery heater system are depicted. The graphs have two verticalaxes. Axes 300 and 400 display the battery voltage measured by voltagemeter 212. Axes 302 and 402 display the temperature of the batterymeasured by transducer 208.

Referring now to FIGS. 3A and 3B, a first embodiment of a method ofoperating an embodiment of the battery heater system is depicted. Inthis method, the electrical heating element is powered cyclically basedon a fixed time duration. The heating element is powered for a fixedtime, and then powered off until the battery recovers to its opencircuit voltage.

The process begins at step 314 with the actuation of the battery heatersystem by a user causing switch 202 to close by pressing button 106 orotherwise activating the battery heater system as described above.Indicator 206 provides feedback to the user that the battery heatersystem has been activated and that it is in operation. The indicator 206may flash or provide different feedback to indicate to the user whendifferent phases of the process are ongoing, such as when the heatingelement is currently active, for example.

In the first method, the heating element 214 is operated for a fixedpower-on time period 308 for each heating cycle and then powered off fora fixed power-off time period 309. The fixed power-on time period 308 isdetermined by an initial discharge cycle of the cold-soaked battery, forexample at −40 degrees Celsius. An initial discharge current iscalculated by dividing the wattage of the heating element 214 by theopen circuit voltage 304 of the battery 214. The electrochemical cells210 are discharged at step 318 at the initial discharge currentdetermined at step 316 through heating element 214. As theelectrochemical cells 210 discharge, electronic controller 200 monitorsthe voltage 305 of the electrochemical cells 210. At step 320, theelectrochemical cells 210 are discharged until the battery voltage 305drops to the discharge cut-off voltage 306 and the elapsed time duringthe discharge step is measured for the power-on time period 308. Then atstep 321 the heater is disconnected from the battery and the time torecover to open circuit voltage is measured for the power-off timeperiod 309. In some embodiments this step may take place undercontrolled circumstances such as a laboratory setting at a selectedtemperature such as −40 degrees C. to determine fixed values to use,which are then stored in the electronic controller 200 for use on anaircraft or inside a production battery.

The discharge cut-off voltage 306 is a predetermined value, which insome embodiments may be 16 volts for a battery with an open circuitvoltage of 26 volts. The discharge cut-off voltage 306 is selected toprevent damage or total discharge of the cold battery. The heatingelement 214 is then shut off by electronic controller 200. The time 308that it took for the battery voltage 305 to drop from voltage 304 tovoltage 306 is the fixed power-on time period 308 to be used by themethod. The fixed power-off time period 309 is similarly determined bymeasuring the time required for the battery voltage 305 to recover fromthe discharge cut-off voltage 306 to the open circuit voltage 304. Insome embodiments, the fixed power-on time period 308 and the fixedpower-off time period 309 are determined experimentally at a selectedtemperature such as −40 degrees C. and then stored as data values foruse in electronic controller 200. In other embodiments, the two timeperiods may be determined during the first discharge cycle as thebattery heater system begins operation. The determination of thepower-on time period and the power-off time period may be performed on atest article of the battery and then used in other similar batteries.

In some embodiments, the heating element is then powered off for thefixed power-off time period 309. This allows the battery voltage 305 torecover to the open circuit voltage and after the power-off time period309 the heater is turned back on for the power-on time period 308. Sincethe electrochemical cells 210 have been heated by the previous period ofheating, the voltage 305 does not drop all the way to voltage 306. Whenthe fixed power-on time period 308 has elapsed, the heating element 214is disconnected and the current flow through element 214 is stopped. Theelectronic controller 200 repeats the cycle of waiting for the fixedpower-off time duration 309, at which time the process may repeat or thebattery heating method may terminate as described in more detail below.

The electronic controller 200 continues to monitor the voltage 305 ofthe electrochemical cells 210. At step 326 the control electronicsdetermines if it is necessary to continue heating the electrochemicalcells 210. If the temperature 311 of the electrochemical cells 210 hasreached the target temperature 312 then the battery heater systemdeactivates. If the temperature 311 of the electrochemical cells 210 hasnot yet reached the target temperature 312, steps 324 and 322 arerepeated. At step 324 the voltage 305 is allowed to recover to opencircuit voltage 304 for the power-off time period, and at step 322 theelectrochemical cells 210 are discharged through the heater 214 for thefixed power-on time duration 308. This cycle is repeated, and in eachcycle the voltage 305 falls less over time 308 as the electrochemicalcells 210 heat up until they reach the target temperature 312. In someembodiments step 324 may take place before step 326. In other words, insome embodiments the controller 200 may allow the battery voltage 305 torecover prior to determining whether to terminate the heating process.In some embodiments, the target temperature 312 is greater than themelting point of the electrolyte used in the electrochemical cells.

In some embodiments of the invention, the power-on time period and thepower-off time period comprise a duty cycle for the heater. The dutycycle may be calculated as the ratio of the power-on time period overthe sum of the power-on time period plus the power-off time period. Thecalculated ratio or percentage may then be applied over a repeatingcycle whose unit time length is equal to the power-on time period plusthe power-off time period.

The electrochemical cells 210 begin at a temperature 311 equal to thecold-soaked temperature 310. Temperature 311 increases due to theheating cycles until it reaches the target temperature 312. In someembodiments the target temperature is 10 degrees Celsius, though thetarget temperature may vary depending on the type of electrochemicalcells 210. The target temperature 312 should be sufficient so thatnormal usage of the electrochemical cells 210 for operation of aircraftsystems will not damage or totally discharge the electrochemical cells210.

Referring now to FIGS. 4A and 4B, a second embodiment of a method ofoperating an embodiment of the battery heater system is depicted. Theprocess begins at step 414 with the actuation of the battery heatersystem by a user causing switch 202 to close by pressing button 106 orotherwise activating the battery heater system as described above.Indicator 206 provides feedback to the user that the battery heatersystem has been activated and that it is in operation. The indicator 206may flash or provide different feedback to indicate to the user whendifferent phases of the process are ongoing, such as when the heatingelement is currently active, for example.

In the second method of operating the battery heater system, the heatingelement 214 is operated periodically at step 416, with each periodcontinuing until the battery voltage 405 drops to the discharge cut-offvoltage 406. Once the battery voltage 405 reaches the discharge cut-offvoltage 406, the heating element 214 is turned off and the batteryvoltage 405 allowed to recover to the open circuit voltage 404. Once thebattery voltage 405 recovers to the open circuit voltage 404, theelectronic controller 200 switches the heating element 214 back on anddischarges it until the battery voltage drops again to the dischargecut-off voltage 406. As the temperature 409 of the electrochemical cells210 increases and the viscosity of the electrolyte decreases, thebattery voltage 405 decreases more slowly during the discharge phase andthe heating periods lengthen, as can be seen by comparing the lengths oftime period 407 and time period 411.

The temperature 409 begins at initial cold-soaked temperature 408 andincreases as a result of the periodic activation of heating element 214until it reaches target temperature 410, at which time the batteryheater system is deactivated by electronic controller 200. Targettemperature 410 is selected so that the normal usage of theelectrochemical cells 210 for operation of aircraft systems will notdamage or totally discharge the electrochemical cells 210.

Referring now to FIGS. 5A and 5B, a graph of the duty cycle of thebattery heater system in a third embodiment of the method of operatingthe battery heater system and a flow diagram of the embodiment of themethod are depicted, respectively. The duty cycle comprises thepercentage of a given time period during which the electrochemical cellsare discharged through the electrical heater. The third embodiment ofthe method activates the heater using a duty cycle for the heatingelement 214 that varies based on the temperature of the electrochemicalcells 210. The duty cycle percentage and duration used by the method fora given temperature are pre-defined based on the characteristics of thebattery and the battery heater system. In one embodiment, the duty cyclemay be defined based on experimental data and testing to prevent thebattery voltage from dropping below the battery discharge cut-offvoltage during a duty cycle. For example and not by way of limitation, acertain battery at −40 degrees C. may be able to maintain voltage abovethe discharge cut-off voltage while discharging current through theheating element for 2.5 seconds (50%) of a 5 second duty cycle, while at−30 degrees C. the battery voltage may remain above the dischargecut-off voltage for 100% of the 5 second duty cycle. In anotherembodiment, a certain battery at −40 degrees C. may be able to maintainvoltage above the discharge cut-off voltage while discharging currentthrough the heating element for 2.2 seconds (55%) of a 4 second dutycycle, while at −20 degrees C. the battery voltage may remain above thedischarge cut-off voltage for 100% of the 4 second duty cycle

One or more temperature thresholds or transitions may be definedproviding for the application of different duty cycle values fordifferent battery temperatures. For example, a threshold temperature 502may be defined whereby the duty cycle below temperature 502 is a firstvalue 500 and the duty cycle at or above temperature 502 is a secondlarger value 506. Another example of a threshold may be at batterytemperatures around the melting temperature of the battery electrolytewhereby the battery heater system operates with a first duty cycle whenoperating below the melting temperature and at a second duty cycle whenoperating at or above the electrolyte melting temperature.

In other embodiments, a single threshold 502 may be provided oradditional thresholds such as 504 may be provided at varyingtemperatures above or below 502. In some embodiments, duty cycle 500 is50% and temperature 502 is −35 degrees C. In some embodiments, dutycycle 506 is 75% and temperature 504 is −30 degrees C. In someembodiments, duty cycle 508 is a 100% duty cycle.

In some embodiments, once the battery temperature reaches a targettemperature 510, the battery heater system may be deactivated. In someembodiments the battery heater system is not immediately turnedcompletely off, but the duty cycle of the heater is decreased until thebattery heater system is deactivated at the heater shut-off temperature516. In the embodiment shown in FIG. 5A, the duty cycle is reduced by anamount 512 for each increase 514 in battery temperature. In someembodiments the duty cycle reduction amount 512 is 10% and thetemperature increase is 1 degree C., causing the duty cycle of thebattery heater system to decrease by 10% for each 1 degree C. above thetarget temperature 510 until the battery heater system is deactivated atthe heater shut-off temperature 516.

Referring now to FIG. 5B, the method of operating an embodiment of thebattery heater system is depicted according to the embodiment shown inFIG. 5A. Once a user initiates the battery heater system in step 518,the heater initially operates using a first duty cycle as describedabove in relation to FIG. 5A. The system monitors the batterytemperature at stop 522, either continuously or periodically, todetermine if the battery temperature has reached a first threshold 502.The temperature monitoring in step 522 may take place in parallel withthe operation of the heater in step 520 and they both may continuesimultaneously with step 522 causing the system to break out of step 520when the target is reached, or the temperature may be checked at aspecific time during each duty cycle before beginning the next dutycycle of heating. The embodiment of the method depicted in FIG. 5B isintended to encompass both continuous and periodic temperaturemonitoring, and parallel or sequential operation of steps 520 and 522.

Once the battery temperature exceeds the target in step 522, the heateris then operated with the second duty cycle such as duty cycle 506 atstep 524. Again, in step 526, the temperature of the battery is checkedand compared to another threshold temperature 504. As described above,steps 524 and 526 may be sequential or operate in parallel. In thedepicted embodiment a second threshold temperature 504 is utilized tochange to a third duty cycle value 508.

In the depicted embodiment and method, once the battery temperaturereaches the second threshold 504 at step 526, step 528 then operates theheater at the third duty cycle value 508. In the depicted embodiment,the value 508 is 100%. In other embodiments, there may only be onethreshold temperature or more than two threshold temperatures, and othervalues of duty cycle may be utilized for 500, 506, and 508. At step 530,electronic controller continues to monitor the battery temperature todetermine if it is above a third threshold. As described above withrespect to other monitoring and heating steps, steps 528 and 530 mayoperate sequentially in a loop or continuously and in parallel. Step 528continues to operate the heater until the battery reaches a targettemperature 510. In some embodiments, target temperature is 10 degreesC., but it may vary in other embodiments depending on the batteryspecification and other circumstances.

In some embodiments, the electronic controller 200 may shut off theheater after the battery reaches the target temperature 510. In otherembodiments, such as those depicted in FIGS. 5A and 5B the electroniccontroller 200 reduces the duty cycle incrementally. In step 532,electronic controller checks to determine if the battery temperature hasincreased by an incremental amount 514. If not, at step 538 electroniccontroller 200 continues to operate the heater at the previous dutycycle. If the battery temperature has increased by at least increment514, the duty cycle is reduced at step 534 by the predetermined amount512. In some embodiments, the increment 514 is 1 degree C. and the dutycycle reduction 512 is 10%. These values may vary in other embodiments.If the reduction in duty cycle causes the duty cycle to reduce to zeroat step 536 then the battery heater system is terminated at step 540. Ifthe duty cycle has not yet reached zero percent, the battery heatersystem continues to operate at step 538. Steps 532 and 538 may operatesequentially or in parallel and continuously.

Referring now to FIG. 5C, another embodiment of the method of operationshown in FIG. 5A is depicted. The method depicted in this figureproduces the same duty cycle profile depicted in FIG. 5A but highlightsthe parallel and continuous nature of the temperature monitoring andheating operations in some embodiments. As with the prior method, a userinitiates the heater at step 542. The electronic controller 200 beginsheating the battery at step 544 using the initial setting for the dutycycle value, such as value 500. This heating process is performedcontinually until interrupted by a later step in the process. Althoughit is continuous the duty cycle value used by the controller 200 tooperate the heater may be altered at other steps in the process withoutstopping the continuous operation of the heater.

While step 544 runs continuously, the electronic controller 200 monitorsthe battery temperature at step 546 to determine if it has reached orexceeded one of the thresholds defined for that embodiment of themethod. As discussed above, the number and value of the thresholds mayvary in different embodiments. If a threshold has been reached thecontroller increases the current duty cycle to the new value, such asvalue 506 at step 550. It may also perform a check at step 548 todetermine if the battery temperature is above the final targettemperature at which heating will be terminated or reduced, such astemperature 510. The heater continues to heat the battery throughoutthis process, and continues to do so as the duty cycle used to operateit is modified by controller 200.

When the battery temperature does reach the target temperature 510, step548 redirects the method to step 552 and causes the duty cycle to bereduced to begin tapering off the heater. In some embodiments the heatermay terminate at temperature 510, but in the depicted embodiment, theduty cycle is reduced at step 552 by a percentage 512 at temperature 510and by another amount 512 for each increase in battery temperature ofvalue 514. This is depicted as steps 552 and 554. If the duty cycle isreduced to zero then step 556 redirects the process to interrupt theoperation of the heater at step 560 and the battery heater system isdeactivated at step 562.

In some embodiments, the various methods described above or otherembodiments may be used in combination in a single battery. For examplethe method depicted in FIGS. 3A and 3B might be used for one temperaturerange, while the method depicted in FIGS. 4A and 4B or FIGS. 5A and 5Bmight be used for another temperature range.

Many different arrangements of the various components depicted, as wellas components not shown, are possible without departing from the spiritand scope of the present invention. Embodiments of the present inventionhave been described with the intent to be illustrative rather thanrestrictive. Alternative embodiments will become apparent to thoseskilled in the art that do not depart from its scope. A skilled artisanmay develop alternative means of implementing the aforementionedimprovements without departing from the scope of the present invention.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations and are contemplated within the scope of the claims. Notall steps listed in the various figures need be carried out in thespecific order described.

What is claimed is:
 1. A method for operating an electrical heater provided in a battery case to heat an electrochemical cell disposed in the battery case, the method comprising the steps of: determining a power-on time period; determining a power-off time period; discharging the electrochemical cell through the electrical heater for the power-on time period; and disconnecting the electrochemical cell from the electrical heater for the power-off time period, wherein the step of determining a power-on time period further comprises: measuring a voltage of the electrochemical cell with a voltage meter; discharging the electrochemical cell through the electrical heater until the voltage of the electrochemical cell drops to a discharge cut-off voltage; and measuring the time period elapsed during the step of discharging until the voltage of the electrochemical cell drops to the discharge cut-off voltage.
 2. The method of claim 1 wherein the step of determining a power-off time period further comprises the steps of: disconnecting the electrochemical cell from the electrical heater when the voltage of the electrochemical cell drops to the discharge cut-off voltage; measuring the time period elapsed until the voltage of the electrochemical cell recovers to an open circuit voltage of the battery cell.
 3. The method of claim 2 wherein the step of determining a power-on time period and the step of determining a power-off time period are executed when the temperature of the electrochemical cell is a selected temperature.
 4. The method of claim 3 wherein the step of determining a power-on time period and the step of determining a power-off time period are executed on a test article of the battery.
 5. The method of claim 2 wherein the step of determining a power-on time period and the step of determining a power-off time period are executed on the battery.
 6. The method of claim 1 further comprising the step of sequentially repeating the steps of discharging and disconnecting until the temperature of the electrochemical cell is at or above a target temperature.
 7. The method of claim 1 wherein the discharge cut-off voltage is selected to prevent damage to the electrochemical cell.
 8. The method of claim 6 wherein the target temperature is greater than the melting point of an electrolyte utilized in the electrochemical cell.
 9. An automated method for operating an electrical heater by a controller, the electrical heater being provided in a battery case to heat an electrochemical cell disposed in the battery case, the method comprising the steps of: selecting a duty cycle comprising a percentage of a unit of time having an initial value; repeatedly discharging the electrochemical cell through the electrical heater for the duty cycle percentage of each unit of time; monitoring the temperature of the electrochemical cell; increasing the percentage of the duty cycle to a first value when the temperature of the electrochemical cell exceeds a first threshold temperature; increasing the percentage of the duty cycle to a second value when the temperature of the electrochemical cell exceeds a second threshold temperature, wherein the second value for the duty cycle is greater than the first value for the duty cycle; and reducing the percentage of the duty cycle when the temperature of the electrochemical cell exceeds a final threshold temperature, wherein the second threshold temperature is greater than the first threshold temperature and less than the final threshold temperature.
 10. The method of claim 9 wherein the step of reducing the percentage of the duty cycle comprises the step of reducing the percentage of the duty cycle by a first incremental value each time the temperature of the electrochemical cell increases by a second incremental value.
 11. The method of claim 10 wherein the first incremental value is 10% and the second incremental value is 1 degree C.
 12. The method of claim 9 wherein the second threshold temperature is equal to or greater than the melting temperature of the electrolyte in the electrochemical cell.
 13. The method of claim 9 wherein the step of selecting a duty cycle comprises the steps of: determining a power-on time period; determining a power-off time period; and calculating the percentage of the duty cycle as the ratio of the power-on time period to the sum of the power-on time period and the power-off time period.
 14. The method of claim 13 wherein the step of determining a power-on time period comprises the steps of: discharging the electrochemical cell through the electrical heater until the voltage of the electrochemical cell drops to a discharge cut-off voltage; and measuring the time period elapsed during the step of discharging until the voltage of the electrochemical cell drops to the discharge cut-off voltage.
 15. The method of claim 14 wherein the step of determining a power-off time period comprises the steps of: disconnecting the electrochemical cell from the electrical heater when the voltage of the electrochemical cell drops to the discharge cut-off voltage; and measuring the time period elapsed until the voltage of the electrochemical cell recovers to an open circuit voltage of the electrochemical cell.
 16. The method of claim 14 wherein the step of determining a power-on time period and the step of determining a power-off time period are executed when the temperature of the electrochemical cell is a selected temperature.
 17. The method of claim 14 wherein the step of determining a power-on time period and the step of determining a power-off time period are executed on a test article of the battery.
 18. The method of claim 9 wherein the first threshold temperature or the second threshold temperature is greater than or equal to the melting point of the electrolyte in the electrochemical cell.
 19. The method of claim 9 wherein the percentage of the duty cycle is selected based on the temperature of the electrochemical cell to prevent damaging the electrochemical cell.
 20. An automated method for heating a battery, the method being performed by a controller, comprising: measuring a voltage and a temperature of an electrochemical cell of the battery; discharging the electrochemical cell through an electrical heater until the voltage of the electrochemical cell drops to a discharge cut-off voltage; disconnecting the electrochemical cell from the electrical heater until the voltage of the electrochemical cell recovers to an open circuit voltage of the battery cell; and repeating the steps of measuring, discharging, and disconnecting until the temperature of the electrochemical cell reaches a target temperature.
 21. An automated heating method for a battery, the method being performed by a controller, comprising: measuring a temperature of the battery; operating a heating element according to a duty cycle to heat the battery using power from an electrochemical cell of the battery; increasing the percentage of the duty cycle to a first value when the temperature of the electrochemical cell exceeds a first threshold temperature; increasing the percentage of the duty cycle to a second value when the temperature of the electrochemical cell exceeds a second threshold temperature, which is greater than the first threshold temperature; and reducing the percentage of the duty cycle when the temperature of the electrochemical cell exceeds a third threshold temperature, which is greater than the second threshold temperature. 